Polypropylene as a Promising Plastic: A Review
Darrell Griffin hates to fly, so it was with no small degree of trepidation that he faced a flight from Los Angeles to New York a couple of years ago. Desperate for a distraction, Griffin, a film producer at the time, turned to his business partner for help. "To take my mind off the flight, he put a film on my handheld PC," Griffin says. The movie was Hercules Unchained, starring Steve Reeves. The diversion worked, and by the time Griffin landed in New York, he had an idea for a new business.
When they returned to L.A., Griffin and his partner sat down to figure out how they could turn that distraction into a business. "Pocket PCs are synced up all the time," Griffin says. "We wanted to figure out how to create a retail product." They devised a technology to compress filmed content from CD-ROMs into a format PDAs could display. Then in November 2001, Griffin helped launch Pocket PC Films, in Sherman Oaks, Calif., to distribute video content for Pocket PC and Palm OS devices. Film fans can buy CD-ROM titles, load them on their computer and sync them into their handheld device. (The films will also run on desktop PCs, but the format remains handheld-size due to licensing issues.)
Griffin, now president of Pocket PC Films, has high hopes. "The market is growing faster than we thought," he says. "Dataquest estimates that 23 million PDAs will be shipped in 2003, but that was before Dell and ViewSonic announced plans to offer models."
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Pocket PC Film now distributes 25,000 titles, priced from $9.99 to $49.99, that run the gamut from 1941’s King of the Zombies to Oliver Stone’s more recent Natural Born Killers. The company also provides self-help and business-oriented content, as well as classic TV packs with shows like Dragnet and the Sports Illustrated’s "Swimsuit 2002" special?the most popular title so far.
High Impact Polystyrene (HIPS), which made of HIPS rigid film, and Rigid Vinyl (sometimes referred to as PVC or RPVC) are often the main films considered for a variety of lower cost applications such as point-of-purchase signage, shelf danglers, membership and loyalty cards, and tags to name a few. On the surface, it is sometimes viewed that the two films are often interchangeable and either one can be used. However, there are some key differences in film characteristics that leads to one film being the better choice over the other depending on the application needs. The chart below compares key characteristics of the two films and will help you make the best choice in using either rigid vinyl or polystyrene film
Plastics are categorized into four main groups: thermoplastics, elastomers, thermosets and polymer compounds. Macromolecular structures distinguish the class of any plastic material as well as its physical properties. Elastomers and thermosets have soft and hard elasticity, respectively; and their resins cannot be melted for recycling purposes. However, thermoplastics are either amorphous or semi-crystalline. Amorphous resins are disordered statistical oriented macromolecules whereas semi-crystalline resins macromolecules are nearly ordered since they are embedded with crystalline phases. Typical amorphous resins are polycarbonate (PC), polystyrene (PS) and polyvinylchloride (PVC) where typical semi-crystalline resins includes polyamide (PA) and polypropylene (PP). Since PP is under the semi-crystalline class, our focus will be on this group.
The annual global demand for plastics reached 245 million tons and it is project to grow dramatically because of the increase in public demand. Figure 2 shows the total plastics demand by 2006. Around 90% of the total demand is accounted for five main commodity plastics: polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS) and polyethylene terephthalate (PET). In 1954, Giulio Natta discovered PP, but commercial production began in 1957. PP is the most widely used thermoplastic since it is very cheap and flexible for molding. The second common plastic type is PE. There are different grades of PE linear low density (LLDPE), low density (LDPE), medium density (MDPE) and high density (HDPE). The density of PE grades ranges between 0.925-0.965 and obviously is higher than PP. However, the uniqueness of PVC film is associated with chlorine content (57 wt. %) in its structure. Annual worldwide PVC demand is approximately 35 million tons.
According to previous studies, plastics are the most recyclable materials and there were 4.4 million tons of plastics recycled in 2006.Polypropylene chemical resistance can be described as follows: excellent resistance to dilute and concentrated acids, alcohols and bases, good resistance to aldehydes, esters, aliphatic hydrocarbons, ketones and limited resistance to aromatic and halogenated hydrocarbons and oxidizing agents. Polypropylene has a crystalline structure with a high level of stiffness and a high melting point compared to other commercial thermoplastics. The Hardness resulted from the methyl groups in its molecular chain structure. Polypropylene is a lightweight polymer with a density of 0.90 g/cm3 that makes it suitable in many industrial applications. Still, polypropylene is not suitable to be used at temperatures below 0°C. Experiments proved that polypropylene has excellent and desirable physical, mechanical, and thermal properties when used in room temperature applications. It is relatively stiff and has a high melting point, low density and relatively good resistance to impacts.
Typical crystallinity of PP is between 40-60%. PP is a low-cost thermoplastic polymer with excellent properties like flame resistance, transparency, high heat distortion temperature, dimensional stability and recyclability making it ideal for a wide range of applications.Propylene is one of the most important industrial petrochemical building blocks used to produce various chemical derivatives. The dominant outlet for propylene is PP which accounts for nearly two-thirds of global propylene consumption as shown in Figure 3. There are three major sources of propylene: from steam cracking of naphtha, gasoline refining process and propane dehydrogenation technology. Polypropylene is the most important material among polyolefins due to three main reasons. First, great properties of PP such as low density, high melting temperature and chemical inertness with low cost making PP optimum for long-life applications. Second, polypropylene is a highly versatile material meaning that diversity in structural designs and mechanical properties are achievable. Third, different morphological structures of PP are possible by using fillers or reinforcing agents and blending PP with other polymers which yield to have superior characteristics. Special and reinforced polypropylene grades include elastomer-modified PP, elastomer modified filled PP, glass fiber-reinforced PP, filled PP, esthetic filled PP, flame-retardant PP and thermoplastic elastomers.
A study shows that branching process of linear Ziegler–Natta polypropylenes is possible by creating chains based on the molecular weight distribution. Introducing branching into a linear polypropylene will result in having a product with high molecular weight, high melt strength and better properties like high modulus and tensile strength, rigidity and excellent heat resistance. Branching parameters β, the average number of branch points per molecule, and λ, the number of branch points per 103 carbons, are calculated to identify the polymer structure and properties.
A post-reactor is used for the branching process. The branching efficiency is associated with chemical composition and temperature of the reaction. The relationship between branching parameters and molecular weight is important to design polymers with unique properties for different applications. PP rigid film is very suitable for reinforcing and filling. Thus, another study focuses on studying the effect of fibers reinforcement on the mechanical properties of fiber-reinforced polypropylene composites (FRPCs).
A composite material is a polymer matrix that is embedded with reinforcement fibers such as glass or carbon. FRPCs have better durability, moisture resistance and high strength properties; making them ideal in applications related to constructions, sports equipment and cars. Synthetic fibers yield to have better mechanical properties compared to natural ones. Glass fibers are widely used as a synthetic reinforcement with PP to produce very good mechanical properties composites. However, mechanical characteristics of composites are controlled by numerous fiber treatments and coupling agents. Fiber diameter is another significant factor in designing composites material. A certain value of fiber diameter thickness should not be exceeded; if so fibers will decrease composites strength.
Electrically conductive PP and PE composites are cost-effective, commercially proven and with outstanding electrical and mechanical properties. It is mandatory to understand the distribution, orientation, interaction and aspect ratio of conductive fillers with the polymer matrix to obtain the required conductivity. Better filler-matrix interactions are achievable at low viscosity, low surface tension and high crystallinity. Grafting technique initiates the interactions and the distribution of the fillers with PP and PE matrices.
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