Polypropylene has numerous benefits such as: low cost, suitable mechanical properties, excellent and chemical and moisture resistance. Moreover, it can be used in various applications like pipe industry, cable, fibers, packing, automotive, sheets and etc.[1]. Also Using a mineral filler such as talc can help decrease the product cost, increase mechanical properties, improve dimensional stability and increase resistance to deformation. The determining factor in the effective performance of talc particles in the polyolefin matrix is making an effective interaction between talc and the polymer matrix and their uniform distribution within the polymer[2]. Due to polar nature of talc and non-polar nature of PP and PE and insufficient compatibility between these two, use a compatibilizer which can improve the interface between filler and matrix, is essential. The most important compatibilizer for improving interaction between carbonate calcium and polyolefin matrix is used, is grafted polypropylene or polyethylene by maleic anhydride (PP/PE-g-MA). PP/PE-g-MA due to its both polar and non-polar parts can make a strong polymer-filler network[3].

Effect of adding PP-g-MA on the mechanical properties of PP/Talc composites

Grafting by increasing the compatibility between PP and talc leads to improving the mechanical properties of PP/Talc composites[4]. For example, adding 3%wt of graft to PP/30%Talc cause increase the tensile strength, modulus, and impact strength. (Figure1)

Fig. 1. Effect of grafting on mechanical properties of PP-Talc composite

Graft has a similar effect on composites containing higher percentages of talc. As in PP/50%Talc composites, as shown in Figure 2, the tensile strength and impact strength increased significantly after the addition of 5%wt grafts.

Fig. 2. Effect of grafting on mechanical properties of PP-Talc compound

By considering the fact that in industry, the cost of products is always important, so a compatibilizer with a low optimum percentage will be able to use in industrial consumption. By adding PP-g-MA high-quality and affordable composites can be produced because its optimal percentages are usually low amounts. For example, at PP/30%Talc composites the optimum graft percentage has been reported 1.5%. (Figure3)

Fig. 3. The optimum amount of Graft in the PP/Talc compound

As it is shown in the Figure4, increasing the percentages of grafts until 25% has not remarkable effects on composite properties.

Fig. 4. Effect of grafting on tensile strength and modulus

Since, one of the weaknesses of PP is its fragility at low temperatures, effect of grafting on impact strength of PP/30%Talc at 0^oC and -20^oC has been investigated. As it is shown in Figure5, samples which contain different percentages of grafting has higher impact strength than the samples without grafting[5].

Fig. 5. Effect of grafting on impact strength

Effect of adding PP-g-MA on rheologic properties of PP/Talc composites

Adding PP-g-Ma to PP/Talc composites, leads to improve flowability and rheologic properties[6].

Addition of PP-g-MA to PP / Talc composites improves flowability and rheological properties, so that the melt flow index (MFI) of PP/30%Talc composites reached 9.4 and 9 g/10min, respectively, by adding 1.5 and 3% grafts, respectively. (Compared to 11 for PP/30%Talc composites without grafts).

Fig. 6. Effect of grafting on processing of PP/Talc

Addition of graft according to increasing the interaction between polypropylene and talc and make as strong network of polymer-filler also can improve the rheologic properties of composites. As all three rheological parameters of complex viscosity, storage modulus and loss modulus of composites showed a significant increase after graft addition. As Figure 7 shows, by adding a graft to the PP / Talc composite, the loss factor is significantly reduced to less than one, resulting in an increase in the elasticity of the system due to better interaction between the polymer and the filler.

Fig. 7. Effect of graft on rheologic properties of polypropylene/talc

Effect of addition PP-g-MA on morphology of PP/Talc composites

The morphological images of talc particles in the PP matrix confirm the mechanical and rheological properties of its composites[7]. Figure 8 shows the distribution morphology of talc particles in PP / 30% Talc composites before and after the addition of 3% graft. After adding 3% graft, the distribution of talc particles has become much more uniform.

Fig. 8. Effect of grafting on the PP/Talc morphology

Comparison of PP / (PP-g-MA) / Talc composites with commercial sample of PP / Talc composites used in automotive industry

One of the most important applications of PP/Talc composites is the use in the automotive industry, especially the interior parts of the car such as the dashboard. Addition of PP-g-MA increases the properties of car parts made of PP/Talc composite. For example, the properties of polypropylene (PP), recycled polypropylene (rPP) and PP/graft/Talc and rPP/graft/Talc composites have been compared with commercial samples of PP/Talc composites (Kinghfa PP), with the same percentages of talc (Figure 9). The formulation of the mentioned samples is according to Table 1.

Table 1. Formulation of samples related to Figure 9

Figure 9 shows the mechanical properties of these samples. As can be seen, using PP-g-MA compatibilizer, even with rPP-based composites, similar properties can be achieved, or in many cases higher than commercial samples.

Fig. 9. Comparison of polypropylene/talc composite containing graft with commercial samples

Author: Emad Izadi Vasafi

References:

1. Maddah, Hisham A. “Polypropylene as a promising plastic: A review.” Am. J. Polym. Sci 6.1 (2016): 1-11.
2. Leong, Y. W., and Abu Bakar. “MB; Ishak, ZAM; Ariffin, A.; Pukánszky, B. Comparison of the mechanical properties and interfacial interactions between talc, kaolin, and calcium carbonate filled polypropylene composites.” J. Appl. Polym. Sci 91 (2004): 3315-3326.
3. Hemmati, M., et al. “Effect of polypropylene–grafted–maleic anhydride compatibilizer on the physical properties of polypropylene/carbon nanotube composites.” Polymers and Polymer Composites 20.6 (2012): 559-566.
4. Liu, Keyan, et al. “Effects of surface modification of talc on mechanical properties of polypropylene/talc composites.” AIP Conference Proceedings. Vol. 1713. No. 1. AIP Publishing LLC, 2016.
5. Ammar, O., et al. “Talc as reinforcing filler in polypropylene compounds: effect on morphology and mechanical properties.” Polym. Sci 3.8 (2017).
6. Alavi, M., et al. “Study of the rheological properties of polypropylene/talc/nanoclay ternary hybrid nano composites.” e-Polymers 10.1 (2010).
7. Denac, Matjaž, Vojko Musil, and Ivan Šmit. “Polypropylene/talc/SEBS (SEBS-g-MA) composites. Part 2. Mechanical properties.” Composites Part A: applied science and manufacturing 36.9 (2005): 1282-1290.

Nowadays, polymeric interlayer adhesives such as polyethylene grafted by maleic anhydride, are used widely for linking polyolefins to polar polymers (e.g., polyamide, epoxy, polyvinyl alcohol and etc.), or metals. These types of adhesives are used as a middle-layer in oil and gas, multi-layer films, multi-layer pipes, pipes junctions, fuel tanks and etc. In this article the application of interlayer adhesives in three-layer coating has been investigated.

Two of the common issues in oil and gas industry are corrosion and rottenness. Fixing and replacing of pipes cause numerous costs. Thus, investigation of producing coatings which can increase resistance to corrosion had always been of interest. Three-layer polyethylene coating is one of the most consumed coatings which has composed from three layers:

1. The bottom layer which is the epoxy primer
2. The middle layer which is grafted polyethylene with maleic anhydride
3. The outer layer which is high density polyethylene (HDPE)

These layers have been showed in Fig 1.

Fig 1. Components of the three-layer anti-corrosion coating of the pipe [1].

How to apply the above three layers with an extruder is shown in Fig 2.

Fig 2. Applying the coating with an extruder [2]

Steps of applying the coating

1. Preparing the surface of steel pipe by grinding (sand blasting)
2. Washing the surface by chromate or phosphoric acid
3. Spraying epoxy powder on the steel surface
4. Apply the outer layer of polyethylene in the form of a strip by extruder

The advantages and disadvantages of polyethylene coating n summary is shown in Table 1.

Table 1. Advantages and disadvantages of polyethylene coating

If the coating suffers from a defect such as a pore or local separation of the coating from the surface, so that corrosive agents such as water and oxygen are allowed to reach the surface, the coating will no longer be able to protect the pipe-line and the pipe will corrode like Fig 3.

Fig 3. A corroded pipe [3]

To reach the maximum protection against corrosion, the coating must have suitable adhesion to the pipe, so it can transfer the stresses which is created from both outside and inside the pipe (such as expansion and contraction due to temperature changes) to the outer layer of polyethylene. So, it can prevent stress on the interface of the pipe and the epoxy layer, followed by separation of the three-layer coating of the pipe.

According to reports which are published at Iran and the other countries, it has been found that most of the times these coatings will be separated from the surface of pipe in a quietly short time (less than five years). That is why it is made concerns about early corrosion of metal pipes.

One of the known issues of polyolefin coatings are low adherence of coating to the surface of metal. Thus, to improve adhesion and protection from corrosion, usually a layer of epoxy resin under the polyolefin coating is used. Although the adhesion of epoxy to metal is appropriate, its adhesion to polyolefins is very weak. To solve the problem one layer of adhesive based on maleic- polyethylene between epoxy layer and outer polyethylene is used. It causes the adhesion of HDPE to metal layer and compatibility of epoxy coating in terms of polar. The adhesive must be compatible with the polymer coating (non-polar) on the one hand and with the epoxy (polar) substrate on the other hand, so a compatibilizer is used in the components of this adhesive. The compatibilizer is polyethylene bonded with maleic anhydride.

The compatible performance of the polyethylene in the interlayer adhesive is remarkably important because the compound links to the bottom layer (epoxy) of its maleic anhydride head due to a chemical reaction between the maleic anhydride and the epoxy group, and the polyethylene head to the outer black layer. The reaction is shown on the Fig. 4.

Fig 4. Chemical reaction between compatibilizer and epoxy layer [4]

The Aria Polymer company is manufacturer of interlayer adhesive for these coatings. For more information call us.

Author: Emad Izadi Vasafi

References:

1. http://m.cnspipes.com/coating-pipe/3pe-steel-pipe/api-5l-gr-b-x42-x60-x70-saw-ssaw-lsaw-er.html

2. https://www.useon.com/3lpe-external-coating-line/

3. https://www.canusacps.com/non_html/reference/TP_01.pdf

4. Ha, Thu Huong, et al. “Maleic anhydride grafted polyethylene powder coated with epoxy resin: A novel reactive hot melt adhesive.” Journal of applied polymer science116.1 (2010): 328-332.

In the article of properties of different types of polymers at polymer films (part one) has mentioned to importance of polymer films at packing industrial and applications of them in this industry. One of the most important categories of polymer films is multilayer films. Significant purposes of combination of polymers in multilayer films is to preserve the quality of food materials and also to prevent them from moisture and oxygen [1].

Based on the features that is said above in this article, polyolefins are considered very useful. Although these materials have some benefits such as: processing, no moisture diffusion, good thermal and mechanical properties, but polyolefins do not have suitable resistance to oxygen diffusion. Unlike the polyolefins, a polar polymer like ethylene vinyl alcohol copolymer (EVOH), due to its chemical structure has resistance against diffusion of oxygen, so this material because of recycling ability and transparency is replacing instead of metalized films and aluminum foils. Based on the data of table1, adding EVOH to the center of multilayer film (the middle layer) leads to remarkable resistance against oxygen in comparison to a single-layer of polyolefins (e.g. LDPE) [1].

Table 1: Comparison of oxygen diffusion rate between single-layer and multi-layer films [2].

However, mixing of polar polymers like EVOH and non-polar polymers such as Polyethylene (PE) because of incompatibility is challenging. That is why to make multi-layer films with this combination need to use of new generation of adhesives which is called interlayer adhesive or Tie layer to promote weak adhesion between the layers.

Properties of packaging films

Packaging films must have four functions:

1. Chemical and thermal stability
2. Moisture resistance and oxygen diffusion resistance
3. High mechanical properties (abrasion, impact and volume stability)
4. Qualified optical properties such as transparency and brilliance

As mentioned above, the main goal of compounding polymers in multi-layer films is preserving the quality of food materials and prevent from exposureing to moisture and oxygen. Due to appropriate properties and resistance against the diffusion of oxygen polyolefins play an essential role in making multi-layer films. Nevertheless, for creating resistance against oxygen diffusion in these multi-layer films it is used from polar polymers such as polyamide or EVOH in the central layer of the films. Fig. 1 shows the regular structure of an ideal film at packaging film industry. Combination of polyolefin layer with the polar polymer layer makes the film resistant in both moisture and oxygen. Such a film with preventing of oxygen diffusion and the other gases would prevent from food spoiling and it can take care of the quality of food with keeping the scent and moisture.

Fig. 1. Function of ideal multi-layer film in food- packaging industrial [3]

With comparison of polymers functions, it can be understood that the resistance of EVOH against oxygen diffusity in 1mm is equal to 10 m of polyethylene [4]. The chart of polymers functions at oxygen diffusion resistance is seen at Fig. 2. Based on the figure, most of the polyolefins because of their chemical structure have weak function against oxygen. Unlike these materials, most of polar polymers like polyvinyl alcohol (PVA), Ethylene vinyl alcohol, PVDC and polyamides which has good function against oxygen diffusion. Between these materials with considering the process conditions, cost and compatibility with environment, EVOH is the best choose for food-packaging as an oxygen diffusion resistance layer.

Fig. 2. Oxygen permeability of some polymers [4]

However, polar polymers (PA, EVOH) and non-polar polymers (PE, PP, PS) have different structure so that they are incompatible. In Fig. 3, compatibility and tendency of different material for each other can be seen.

Fig. 3. Combability of different materials [5].

So, to mixing the incompatible polymers in different layers, it is needed to a thin layer of reaction or non-reaction adhesive whit dual functionality to increase the adhesion strength between incompatible layers according to the Fig. 4.

Fig. 4. Adhesion strength between layers [5].

The difference between these adhesives with regular adhesives is because of adhesion mechanism. This adhesive due to their components would be able to build chemical interaction (in adhesive joint surface/ polar layer of polymer) according to Fig. 5. Hence, chemical surface forces such as covalent and hydrogenic forces will be able to make adhesion between adhesive and polymer.

Fig. 5. Interaction of adhesive and polar polymer mechanism (covalent bonding) [6].

To produce multi-layer films, co-extrusion process is considered the best one. At this process two or more polymer are extruded together which is resulted to a composite film. This can lead to creating a film whit different function. According to Fig. 6, to make the film by this process two methods can be used: Casting and Blowing.

Fig. 6. Methods of producing multi-layer films by co-extrusion process [6].

Due to the process and components of producing the adhesive, different factors can affect on interlayer adhesion. For example, increasing temperature and time in the producing process helps the increasing of thickness of adhesive and amount of functionality help more chemical interaction. In addition, every process at producing level which is led to orientation at adhesive chains, will declined adhesion strength. The components of interlayer adhesives must be designed in a way that this decrease by orientation be the least possible. Effects of these factors have been specified in Fig. 7.

Fig. 7. Changes in adhesion by different factors [7].

These adhesives can be used in different categories and different applications. For example, suitable packaging films for some food material are mentioned at Table 2.

Table 2. Suitable packaging film for various food grade.

Finally, to select an appropriate adhesive, we should consider the layers of a film, the strength which is needed, physical requirements, cost and processing factors. In addition, interlayer adhesive components must be designed in a way to make a good balance between adhesion with transparency and flexibility.

Fig. 8. Non-flexible films (on the left) and resistant films against oxygen diffusity (on the right) [8].

Aria Polymer Pishgam is pioneer at manufacturing of interlayer adhesives for different kinds of layers. To get to know these adhesives, please call to Aria Polymer experts.

Author: Emad Izadi Vasafi

References

[1] Maes, C. Luyten, W., Updates on the Barrier Properties of Ethylene Vinyl Alcohol Copolymer (EVOH): A Review. Polymer Reviews 2018, 58, 209–246.

[2] Sangaj, N.S. and V. Malshe, Permeability of polymers in protective organic coatings. Progress in Organic coatings, 2004. 50(1): p. 28-39.

[3] Mokwena, K.K. and J. Tang, Ethylene vinyl alcohol: a review of barrier properties for packaging shelf stable foods. Critical reviews in food science and nutrition, 2012. 52(7): p. 640-650.

[4] Morris, B.A., Tie layer technology for multilayer coextrusion of single-use biopharma bags. 2017.

[5] Poisson, C., et al., Optimization of PE/binder/PA extrusion blow‐molded films. II. Adhesion properties improvement using binder/EVA blends. Journal of applied polymer science, 2006. 101(1): p. 118-127.

[6] Ebnesajjad, S. and C. Ebnesajjad, Surface treatment of materials for adhesive bonding. 2013: William Andrew.

[7] Butler, T.I. and B.A. Morris, PE-based multilayer film structures, in Multilayer flexible packaging. 2016, Elsevier. p. 281-310.

[8] Kamykowski, G.W., Factors affecting adhesion of tie layers between polypropylene and polyamides. Journal of Plastic Film & Sheeting, 2000. 16(3): p. 237-246.

Today, more than 70% of global POM production capacity is centered in Asia. The production market has been growing approximately 4.5% annually and a total of 1350 kiloton in 2018. The main buyers are the automotive and electronics industries. Polyoxymethylene was discovered by Hermann Staudinger, a German chemist, in 1920. But due to stability and thermal problems, this polymer was not commercialized at that time. This polymer was marketed in 1956 by the American company DuPont as a homopolymer under the brand name Delrin for the first time. Polyacetal is an engineering polymer with a wide range of applications. Polyacetal’s properties are similar to metals. Due to its special fatigue and abrasion properties, Polyacetal as an industrial polymer has been able to fill the vacancies of some metals in various industries, especially automobiles. This polymer has excellent properties that fill the gap between general polymers and metals [1]. Since the introduction of this polymer into the industry, we have seen its wide applications – such as applications in the automotive industry, electrical equipment, construction, electronics, and many other industries.

Properties of Polyacetal (POM)

Polyoxymethylene (POM), polyformaldehyde or acetate, is one of the engineering thermoplastics that have excellent properties. Polyacetal is a crystalline polymer with high performance, excellent friction, abrasion properties, dimensional and chemical stability. All of these make it used widely for the production of automotive slippers in the automotive electronics machinery industry. [2]

Thermo-Physical Properties of Polyacetal [3].

Due to the low temperature, polyaldehydes are generally unstable at ambient and high temperature and easily get de polymerized. For this reason, most polyaldehydes, such as poly (acetaldehyde) and polybutyraldehyde, have little or no commercial use. In this case, formaldehyde is an exception. Its temperature is significantly higher than other polyacetals. The temperature at which polymer is depolymerized can be increased by converting lower stable hydroxyl groups to stable ester groups, for example by reacting with anhydrides. This reaction is called the final blockage or final coating. The stability of polyacetales can also be improved by copolymerization with other monomers. The main method is cyclic copromerization of trioxane (formaldehyde cyclic trimmer) with a small amount of cyclic ether (usually ethylene oxide or 3.1-dioxolan):

Another disadvantage of polyacetal is its poor UV stability. Prolonged UV radiation causes damage and leads to discoloration, brittleness, and loss of strength. To improve the stability of UV rays, amine light stabilizers and UV absorbers are added to the mixture. Some pigments, such as carbon black, also provide some protection against UV rays.POM and its copolymers are often a great choice for applications that required low friction, high impact, and impact resistance. It can be processed with all the common methods used for thermoplastics. It can be economically injected into very complex parts or it can be extruded on rods, tubes, profiles, and sheets. They are often done with cutting tools used for producing machined parts (with high precision)[3].

Applications of Polyoxymethylene (POM)

Polyoxymethylene is ideal for applications which high strength and durability are important. Injectable POM components are used in a wide range of plastic products [4] such as:

Polyoxymethylene (POM), also known as poly (methylene oxide), is a thermoplastic engineering family (ETPs) that has excellent mechanical properties and high chemical resistance. These properties make POM suitable in a wide range of industry sectors (e.g. automotive, machinery, electricity, and electronics). The share of each market is shown in the chart below [5].

Different Grades of Polyacetal in Iran and Other Countries

POM grades are often produced with varying degrees of polymerization, resulting in different properties for responding to applications. The various forms of POM resins are stated below:

1. Standard / unreinforced grades

2. Reinforced grades: Glass fibers, carbon fibers or POM grades reinforced with hollow glass fiber show high tensile strength or stiffness depending on the type and amount of polymer reinforcement.

3. High degree of impact and resistance: The combination of POM resins with rubber, TPU, and other polymers such as POE, etc lead to mixtures with higher impact resistance.

4. High slip/abrasion grade: Modification of POM resins with additives such as graphite, PTFE, mineral fillers, etc. increases the slip resistance properties.

5. UV-stabilized grades: Ultraviolet stabilizers, such as amine barrier light stabilizers and UV absorbers, are often added to POM resins or mixtures to improve UV stability.

6. Nanocomposites: Additives such as CNT, ZnO, etc. are used to produce POM nanocomposites.

7. Other grades: Adding Al or bronze powder increases the electrical conductivity or thermal distortion point of POM resins. Fluorocarbons lead to good surface slipping in polyacetal and prevent cracking.

Some commercial-grade and POM suppliers

Fiberglass Reinforced Polyacetal: [6]

An Overview of the Future

Polyoxymethylene is a polymer that its properties depend on many factors during the production, processing, and production of final goods. Expanding knowledge about the effects of production conditions on product characteristics improves the mechanical and chemical properties of the polymer significantly. Using better stabilizers and formaldehyde adsorbents is effective in improving the functional properties of the polymer as well as increasing the service life of the products. Improvement of the mechanical properties of the polymer is so important in development. It will most likely be possible through the preparation of composites, including nanocomposites that show better strength. [5]

منابع

[1] https://en.kunststoffe.de/a/specialistarticle/polyoxymethylene-pom-245435         

[2] Pielichowska, K. (2015). Preparation and characterization of polyoxymethylene nanocomposites. In Manufacturing of nanocomposites with engineering plastics (pp. 103-125). Wood head Publishing

[3]https://polymerdatabase.com

[4] https://www.retlawindustries.com 

 [5] Tokarz, L., Pawlowski, S., & Kedzierski, M. (2014). Polyoxymethylene Applications. Polyoxymethylene handbook: structure, properties, applications and their nanocomposites, 153 -161.

[6] https://omnexus.specialchem.com

Polycarbonates (PC) are condensation polymers having desirable properties such as high clarity, thermal stability, high heat distortion temperature, and in spite of their hardness, they are flexible instead of being brittle against impact. Having such properties placed them in engineering polymer groups. Polycarbonates are easily formed and different grades of them are obtained by processes such as extrusion, injection, and blow molding. So far, the most consumed polycarbonates, which are used in more than 90% of commercial usage, are produced from the reaction between bisphenol A and a dysfunctional, proton-accepting species such as diphenyl carbonate or phosgene. The common chemical structure of this kind of polycarbonate is shown in figure1 [1].

Figure 1: Chemical Structure of Polycarbonate Based On Bisphenol A. [1]

Bisphenol A, also called 2,2 Bis(4-hydroxyphenyl) propane as its official chemical name, is a dysfunctional monomer with two reactive hydroxyl groups (fig 2b) which polymerized with dicarbonyl monomers, such as phosgene or diphenyl carbonate (fig 2a). During the polymerization process, the hydroxyl group of bisphenol A deprotonate in a base condition, and then the oxygen atoms on the bisphenol A residue form ester bonds with dicarbonyl compounds (fig3). The polymerization process will be over when a monohydric phenol reacts with the end of the growing chain[1].

Fig.2: A) Phosgene B) Bisphenol A Monomer [1]

Fig 3: Polycarbonate Polymerization Based On Phosgene and Bisphenol A. [1]

Polycarbonates used in common manufacturing processes are not crystalline. Evidence from the local orientation of chains at the molecular level of polycarbonates suggests that repeating unites can be reassembled onto chains in a structure similar to letter Z. But these large-scale monomeric units do not associate with each other to form a regular crystalline material; and only under specific conditions, such as slow cooling of the polymer, small crystalline domains can be created. Since these conditions are not met in commercial processes, it is easy to say that generally, all the polycarbonates used in industry have an amorphous structure.

Polycarbonates can be manufactured by interfacial polymerization or through a melt esterification process in both batch or continuous conditions. The properties of polycarbonates can differ by different polymerization methods. For example, polycarbonates manufactured via interfacial polymerization are more stable in high temperatures and are less stiff than those produced via melt esterification. So while choosing a grade of polycarbonate resin for a particular application, it is crucial to know the method by which the polymer is produced.

Typically, polycarbonates at a wide range of temperatures display high impact resistance, good thermal stability, excellent clarity and high modulus. For these reasons, they are used in many applications with challenging performance requirements, such as fire fighter’s helmets, power tools, and appliance housings, face shields, and automotive and aircraft panels; and they have received special attention compared to other unmodified engineering plastics (table 1). Negative aspects of polycarbonates are photo-degradation during exposure to ultraviolet and gamma radiation, low chemical resistance, and susceptibility to crazing especially when exposed to solvents, mechanical stresses, or high-temperature conditions [4].

Table 1: Comparison on Properties of Some Engineering Thermoplastics. [4]

The high modulus of polycarbonates even under high-temperature conditions is the result of the high glass transition temperature(Tg) of the polymer (141-150 °c). Under this temperature, the polymer is rigid and displays little distortion under load. By increasing glass transition temperature, the modulus can be improved in high temperatures, which can be achieved by a sensible selection of bisphenol. For increasing glass transition temperature, substituents can be added to phenol groups. In this way, by adding only one functional group to the phenol ring in the bisphenol, the Tg decreases. But by adding the second functional group to the ring, the segmental bonds are reduced and the Tg increases. Modification of bisphenol structure can also affect the stiffness of the final polymer. For example, the structure shown in figure 4 connects the two phenol rings to each other. This inflexible molecular structure creates a rigid polycarbonate backbone resulting in a stiffer polymer. So generally it can be concluded that the stiffness of the polymer increases by increasing the rigidity of a bisphenol section[1].

Fig.4: Chemical Structure of Spirodilactam Bisphenol.

The high clarity of polycarbonate is the result of its amorphous structure. There are no crystalline/amorphous interfaces that can scatter light which leads to opacity. Polycarbonate’s refractive index differs a little from glass, so it makes Polycarbonate a perfect substitute for glass. For example, drawers used in refrigerators are made of polycarbonate whose transparency, lightweight, and structural integrity mimic the appearance of glass similar to polycarbonate.

Polycarbonate has a unique birefringence behavior due to the orientation of small-scale polymer chains in injection molding components. It is seen as a rainbow effect in which the spectrum of colors is seen at certain angles. Injection-molded materials cause this phenomenon due to having highly ordered anisotropic skin.

Polycarbonates are resistant to alcohols, ordinary soaps, some oils, and gases, and dilute acids. But they are not resistant to dilute and strong bases, chlorinated solvents, organic ketones, and cyclic ethers (table 2). They can also crack parts if exposed to fatty acids as well as alcohol at high pressures [2].

Table 2: Chemical Resistance of Some Engineering Thermoplastics. [2]

Table 3: Some of the Physical and Mechanical Properties of PCs. [3]

Applications of Polycarbonate:

Polycarbonates can be used in manufacturing CD/DVD, glasses, contact lenses, bulletproof windows, medical applications such as surgical instruments- drug delivery systems- hemodialysis membranes- blood vessels- blood filters, LED screens, car headlight housing. They are also used in direct contact with foods and beverages due to their heat resistance, crush resistance, and ability to match with the health regulations. Food containers made of polycarbonate are reusable, help to keep freshness, protect food from contamination, and can be easily used in refrigerators or microwaves.

Table 4: Samples of Polycarbonate Grades in Khuzestan Petrochemical Company. [5]

Currently, the main production of polycarbonate in Iran belongs to the Khuzestan petrochemical company. This company is the main supplier of raw materials used in industries mentioned above by producing PCs with different properties. Examples of different grades produced by this company are mentioned in the table below [5].

Author: Hanieh Tavakoli

Nowadays a combination of different physical, mechanical, and thermal properties is needed in many engineering applications, but it is not possible to use only one type of material that creates all desirable properties. For example, in the automotive industry such as dashboard production, we need a compound having proper stretch stability and anti-strength, hit resistance, and resistance to atmospheric conditions such as light (UV) and strengthen themselves even at high temperatures.

Since it is not possible to find a material that has all of these properties at the same time, there must be a method for combining the properties of the materials. This technique is called polymer blending. Polymer compounds mainly consist of one or more non-disperse phases in a continuous phase (matrix). The discontinuous phase is usually stronger than the continuous phase. Therefore, it is called the reinforcing phase, such as fibers, nano clay fillers, carbon nanotubes, graphene, etc.

Often in the application of polymer compounds, there are many problems related to their stability and durability. Especially considering that the durability of these materials in the worst conditions of long-term use is 10 or even 40 years. These problems are related to the environmental conditions used and the type of use (including maintenance, repair, and modification). The stability and durability of each sample is the main destination in the economy of different industries.

In some cases, only shortly after being in an environmental unbelievable situation, a catastrophic defect in the typical structural attachment appears. Ageing is one of the main challenges in applications such as polymer insulation. The main topic in ageing includes predicting how, when and how quickly defects occur, as well as conditions that can result in defects.                                                              

Ageing is a term used in many branches of polymer science and engineering when the properties of the polymer change over a period of time (in the case of mechanical properties or visual colour change.) These changes may be seen in mechanical properties such as tensile strength and toughness, in physical properties such as density, or in chemical properties such as reactions to corrosive chemicals. The source of these changes may be independent of the environment, or they may be of chemical origins, such as the gradual curing of a thermostatic material (vulcanization process), or of physical origins, such as rapid cooling of the polymer under volumetric release conditions. In other cases, the changes may be the result of interaction with the environment, such as when oxidation leads to chain failure. In this article, different types of ageing polymers are analyzed and the main sections related to physical ageing, thermal ageing, and weathering are also mentioned.

Types of Ageing

1)Physical Ageing:

Physical ageing is one of the most common types of ageing. This type of ageing usually happens alongside other ageing described below. For this reason, it is necessary to deal with this type of ageing first [1]. Physical ageing is one of the classifications of ageing of polymeric materials, which usually increases with increasing density and decreasing molecular structural energy of the semi-crystalline or amorphous material when it is undergoing long-term transport [2].

Figure 1- Density Diagram Against Time Related to Physical Ageing [4].

Figure 2. Examples Of Polymer Degradation Under Physical Conditions.        

2)Thermal Ageing:

At high temperatures, if a destructive chemical agent (often oxygen) is applied to a polymer, it causes chemical reactions. These reactions can occur slowly or even don’t happen at ambient temperature. Over these conditions, they are collectively referred to as “heat degradation” and this is a phenomenon studied extensively in polymer science. In some cases, the use of polymers in various applications such as pressure molding to produce suitable parts can be considered.

It is worth mentioning that the thermal stability of the polymer should be improved by using appropriate thermal stabilizers to prevent polymer degradation during high temperatures.[1]

Figure 3. Examples of Degradation of Polymers Against Environmental Condition.

3)Weathering :

Weathering is sometimes referred to as natural ageing. Outdoor polymers can be damaged by several factors including UV rays, water, pollutants (in the form of gases or acid rain), rising temperatures, and temperature changes. In many cases, the main cause of damage is oxidation, which begins with ultraviolet radiation. As a result, there are many tests for ageing caused by sunlight that test the durability of the polymer. Stabilizers are also available for polymer weather resistance. The most common artificial sources of ultraviolet light are xenon lamps and fluorescent tubes [1]. UV rays (UV) radiation has enough energy to destroy the chemical bonds of various polymers such as polyolefin, polyester, polystyrene, and. Ultraviolet damage is intensified and accelerated in the presence of heat and invading fluids. The energy of this ray varies according to the amount of cloudiness in different geographical areas. To prevent this degradation, absorbent and UV-stabilizing additives, UVA and HALS, respectively, have been commercialized. The first group, UVA, absorbs UV rays and converts them to heat. The second group stabilizes the polymer against this radiation, not by adsorption, but by stopping the polymer degradation reaction.

Figure 4 – Mechanism of Action of Antiviral Additives in Polymers[3].

The used absorbents are suitable for preventing the degradation of the polymer mass, pigments, and other sensitive additives used in the polymer, and for protecting the polymer matrix itself (for example, in packaging). On the other hand, the HALS family is suitable for stabilizing parts with high surfaces and low thickness, such as polymer films. Concomitant use of UVA and HALS has a synergistic effect in many applications, and usually, such anti-UV additives are used in certain proportions in masterbatches.

When choosing the optimal composition for stabilizing the polymer against light, factors such as the type of polymer, the thickness of the part produced, the presence of other additives, especially pigments, should be considered. UV light stabilizing additives are used in masterbatch in many applications such as auto parts, agricultural and greenhouse films, fibers, textiles, and other polymer parts that are exposed to sunlight [3].

Figure (5) – An Example of a UV Testing Device Used to Test Weather Resistance [4].

Author: Maryam Momeni

References:

[1] White, J. R. (2006). Polymer ageing: physics, chemistry or engineering? Time to reflect. Comptes Rendus Chimie, 9(11-12), 1396-1408.

[2]  HUTCHINSON, John M. Physical ageing of polymers. Progress in polymer science, 1995, 20.4: 703-760.‏                              

[3] https://cheminews.ir/fa/2020/03/article-8676

[4] Dias, MH Motta, et al. “Effect of fiber-matrix adhesion on the creep behavior of CF/PPS composites: temperature and physical ageing characterization.” Mechanics of time-dependent materials 20.2 (2016): 245-262.

As explained in previous articles, compounds containing flame retardant additives are used  due to various reasons such as less smoke production, no corrosive gases, excellent dielectric properties and also the possibility of recycling in various applications such as power cables and etc. [1]. One of the main disadvantages of this group of materials is their incompatibility with different polymer bases such as polyolefins, which of course can be improved by using enough and suitable coupling agent[2].

Figure (1) – Application of Polymeric Compounds Containing Flame Retardant Compounds Used in The Power Cables Industry.

In this article, in order to analyze the effect of suitable coupling agent, the effect of two different types of coupling agents including maleic anhydride based coupling agent and Ethylene Acrylate terpolymer in a certain amount (5% by weight) are discussed. Generally, coupling agents used in the polymer processing industry are a variety of coupling agent produced in reactive extrusion processes or various polymerization reactors. but choosing the appropriate type of coupling agent depends on the process of compound production, type of mixture and the polymer properties.

Figure 2-A) Ethylene-Acrylate-Maleic Anhydride Terpolymer Coupling Agent 1b) Polyethylene- G- Maleic Anhydride [3].

Figure (3-6) shows the changes in MFI and mechanical properties in one of the common polymer compounds in the production of power cables with EVA/LLDPE /ATH formulation and in the presence of constant values ​​of two types of maleic anhydride and terpolymer coupling agent (5% by weight of LLDPE-g-MA and Ethylene / Acrylate / MA terpolymer coupling agent). As shown in this set of figures, the presence of coupling agent has caused an increase in mechanical properties, although this improvement in properties has occurred at a higher level in the presence of coupling agent.

In general, regarding the mechanism of coupling agents, it can be concluded that in a polymer compound, its organic part can react with the polymer matrix, and the functional polar part of the coupling agent reacts with hydroxyl groups on the surface of the flame retardant substance, such as ATH. This process by improving the adhesion between the components, increases tensile strength and elongation at break significantly.

Figure (3) – Comparison of The MFI Of EVA / LLDPE / ATH Compound in the Presence Of 5% Wt Of Two Types of Maleic Anhydride and Terpolymer Coupling Agents. [4]

using an acrylate terpolymer coupling agent causes a greater increase in the MFI of the compound in comparison to using a maleic anhydride based coupling agent due to a greater drop in the viscosity of the compound in the presence of the acrylic groups in its chemical structure.

Figure (4) – Comparison of Tensile Strength of EVA / LLDPE / ATH Compound in The Presence Of 5% Wt. Of  Two Types Of Maleic Anhydride And Terpolymer Based Coupling Agent [4].

Figure (5) – Comparison of The Elongation at The Break Of 5% Wt. Of EVA / LLDPE / ATH Compound in The Presence of Two Types of Maleic Anhydride and Terpolymer Coupling Agents [4].

As shown in above diagrams, the use of both types of coupling agents improves the mechanical properties of the compound such as tensile strength and elongation at break. Due to the higher yield of maleic anhydride in the desired polymer compound, a higher degree of improvement in mechanical properties has been achieved by the LLDPE-g-MA coupling agent.

Figure (6) – Evaluation of The Limit Oxygen Index (LOI) Of EVA / LLDPE / ATH in the Presence of A Constant Amount of Two Types of Coupling Agent [4].

As it is shown in figure (6), by using of coupling agent based on maleic anhydride, the amount of oxygen index has increased slightly and as a result, the flammability of the polymer compound has been greatly reduced.

Author: Ali Moshkriz

Polymers containing the flame-retardant substance are compounds used as an alternative to PVC based compounds in cables and other flame retardancy polymer compounds [1]. The most important feature of these compounds is that they emit less toxic smoke and gas, in addition to the property of flame retardancy [2]. the additives used in such cables are mainly mineral and have polar substances such as magnesium hydroxide and aluminum trihydrate or magnesium dihydrate, etc. so when they are combined with a non-polar polymer phase such as polyolefins, due to insufficient compatibility with the desired matrix, the compound suffers from the phenomenon of the phase separation. because of  a sharp increase in melt viscosity and subsequent increasing torque, the production of such compounds is challengeable. All these factors together cause a sample with poor mechanical properties and low processability [3]. In such compounds, it is necessary to use a compatibilizer with high and efficient performance to create sufficient interaction at the interfacial boundary between a non-polar polymeric material and polar mineral substance to improve the joint surface of the filler and polymer matrix by creating appropriate chemical mechanisms to obtain optimum rheological and enhance processability of the polymer compounds [4].

Figure (1) – Some Applications of HFFR Compounds.

As discussed in the previous article, the use of appropriate coupling agents such as LLDPE-g-MA at a certain extent can have a significant effect on improving the properties of compounds containing mineral filler (HFFR). In this article, the main purpose is to compare two types of compatibilizers at different levels of maleic anhydride grafting content.  For this purpose, a polymer blend with the base of EVA/ LLDPE /ATH with 60% of HFFR by weight, in the presence of two types of coupling agents with medium grafting level between 0.7-1 and with high grafting level in the range of 1.3-1 were tested.

One of the key parameters in the production of flame retardant polymer blends is the measurement of the limit oxygen index (LOI); the larger this parameter is, the higher performance of the blend in flame retardation is achieved. In Figure (2), this index is examined for the desired blend and to investigate the effect of LLDPE-g-MA compatibilizer at different grafting levels.

Figure (2) – Comparison of The LOI Index of the EVA/LLDPE /ATH Compound in The Presence of Two Type               Coupling Agents with Different Grafting Levels [5].

Another parameter examined to compare the properties of this polymer compound is the MFI test; the results of two samples are given in Figure (3). As the results show, the use of a coupling agent with a higher grafting level leads to an HFFR compound with a higher MFI.

Figure (3) – Comparison the MFI Index of The Compound in The Presence of Two Compatibilizer Samples                     with Different Grafting Levels [5].

Another important parameter in polymer compounds, is the study of mechanical properties of the blend, such as elongation at break and tensile strength.  The following diagrams show the amount of tensile strength and elongation at the break of the samples in the presence of two types of coupling agents at different grafting level.

Figure (4) – Comparison of Changes in Tensile Strength of EVA / LLDPE / ATH Blend in The Presence of Two Coupling Agents with Different Grafting Levels [5].

Figure (6) – Comparison of Changes in Elongation at Break of EVA / LLDPE / ATH in the Presence of Two Coupling Agents with Different Grafting Levels [5].

In most polymer compounds, the higher percentage of reactant functional groups in the coupling agent causes the greater ability of this type of coupling agent to establish sufficient interaction between the filler particles (polar element) and the polymer matrix (non-polar element). As shown in figure (4-5) by increasing the grafting level from medium to high, the mechanical properties of the compounds increase significantly. It is clear that the use of coupling agent with higher grafting level, has led to an increase in tensile strength at a higher level, and also its MFI has increased more. Improving mechanical properties such as tensile strength due to better reinforcement of the interfacial adhesion between the filler particles and the polymer matrix, this enhancement is accompanied by a simultaneous increase in the MFI compound.

Aria Polymer Pishgam Company has presented its coupling agent product based on functionalized linear low density polyethylene (LLDPE-g-MA), especially for using in HFFR compounds. For more information about this product and other related products, please contact the sales department of Aria Polymer.

Author: Ali Moshkriz