Comparison of PP and PET bottle blowing

Polypropylene is a strong competitor to polyethylene terephthalate (PET) for blown bottle applications for several reasons: Compared to PET, polypropylene is less expensive, lighter, and more durable Hot fill and lower moisture vapor permeability; a clear polypropylene with higher clarity and gloss compared to PET.

On the other hand, the recycling time of polypropylene bottles is longer than that of PET bottles, and polypropylene has a narrower processing temperature compared with PET; the gas barrier properties of polypropylene are not as good as PET, and the flexibility is not as good as PET, but Producers of polypropylene resins and manufacturers of additives have taken a big step toward overcoming these shortcomings.

Although the unit price of polypropylene is lower than that of polyethylene terephthalate, adding fillers, additional processing or adding barrier layers to improve the competitiveness of polypropylene relative to PET may reduce its low price advantage. But compared to PET, PP is a more economical choice as a packaging material in many applications.

Due to the dialysis of polypropylene to carbon dioxide, polypropylene cannot replace PET as a beverage packaging material; however, compared with PET bottles, blown polypropylene bottles have made some progress in packaging water, hot juice, iced tea and sports drinks. Polypropylene bottles are becoming packaging materials for auxiliary materials, food ingredients, flavorings and other cooked foods, as well as packaging bottles for detergents and cleaning agents. The ever-expanding food containers are a favorable prospect for the development of polypropylene in the future.

Figure 1: Considering cost-effectiveness, the clear water-filled polypropylene bottle on the left is the best choice next to the PET bottle.

Compared with polyethylene terephthalate, the price of polypropylene is 10-30% cheaper, and its density is lower than that of PET (0.9 g/cc vs. 1.35 g/cc), so the quality of polypropylene bottles is lower than that of polyethylene terephthalate Ethylene glycol diformate is light in weight. Polypropylene bottles are hot-fillable at 100°C, if juice and jam products are to be used at these temperatures. In contrast ordinary PET bottles cannot be filled above the glass transition temperature of 76°C (however, some recent enhancements have allowed PET to be filled at higher temperatures).

Figure 2: Bottles made of copolymers of atactic polypropylene can be hot-filled at 100°C without any permanent deformation

The barrier performance of polypropylene to moisture is about 5 times that of PET, but the permeability of PP to oxygen and carbon dioxide is about 30 times that of PET. Therefore, PP bottles need a barrier layer to prevent gas from entering or escaping from it, while PET bottles do not require this barrier layer.

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In pure state, PP is not as transparent as PET, so PP needs to add transparent additives to make its transparency similar to that of PET; compared with PET, PP has low thermal conductivity, so it takes a longer time to heat or heat during processing. Cool PP, so the production cycle time of PP bottles is 25% longer than that of PET bottles; the nucleating agent can accelerate the crystallization during the cooling process and shorten the cycle time, so shortening the cycle time can improve transparency.

The processing temperature range of PP is only 3-5°C, while the processing temperature range of PET is 10-15°C; when producing blow-molded bottles, the equipment of PP preform production line is compared with that of PET preform production line Not that expensive.

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PP bottles produced by extrusion blow molding can be used to package ketchup and flavorings, cleaning products, mineral water, food and beverages

Figure 3: Extrusion blow molded PP bottles have a handle that cannot be used in PET injection blow molding processes.

PP bottles produced by stretch blow molding are mainly used for the packaging of water, medicines, dried food and flavorings, household cleaners, isobaric water and sports drinks, children's products, household appliances, liquid soaps and detergents.

PP PET blow molding comparison

Disadvantages of PP

1. PP is more difficult to absorb and disperse heat than PET, and the bottle wall of the OPP preform is thicker (in order to achieve the same rigidity, the OPP bottle needs to be thicker than the bottle wall of the same PET bottle). Therefore, although the processing temperature of PP in the preform injection molding stage is lower (230°C), the blow molding of OPP bottles needs to be completed at 130°C, which not only consumes more energy than PET plastic bottles, but also the molding and cooling cycles It is also longer, causing the yield of OPP to be 25% lower than that of PET.

2. The processing temperature for OPP stretch blow molding is narrow, only 5°C, while PET is 15°C, which increases the probability of parking and waste products. In addition, the stretching method of OPP is also different. This is because certain parts of the preform, such as the base and neck, may be less oriented than the body of the bottle and more likely to cause fogging. Therefore, designers of preforms and OPP bottles must optimize orientation and stretch ratios in order to achieve OPP clarity close to that of PET.

3. The lower rigidity of OPP means that OPP bottles have lower top load and lower strength than PET bottles. To achieve the same rigidity, OPP bottles need to have thicker walls than equivalent PET bottles.

At present, many companies have improved some of the shortcomings of OPP bottles through research and development of PP. For example, 3988 can solve problems including haze (mainly bottle neck and bottle bottom), long preform molding cycle (solved by enhancing PP nucleation), and poor processing stability during blowing and drawing.

OPP bottles have excellent heat resistance and can withstand a temperature of 205°C during hot filling, while non-heat-resistant PET bottles can only withstand a temperature of 160°C to 170°C during hot filling. In addition, PET bottles also need to use vacuum trays to prevent collapse when the hot filling temperature exceeds 180°C. When the bottle wall becomes thicker, the production speed slows down, resulting in a 20% increase in production costs. Single-layer OPP bottles can be used in a variety of hot-filling food markets, especially when the hot-filling temperature is high (180 ° C ~ 205 ° C), so as to serve fruit juice, jam, pectin, pickled products, sauces, etc. Barrier and non-barrier packaging for foods such as juice and ketchup has opened up the market. OPP bottles are cheap. Both PET bottles and OPP bottles need to add a gas barrier layer. Although the cost of the barrier materials used in PET bottles and OPP bottles is basically the same, the price advantage of OPP bottles makes them more attractive in the gas barrier plastic bottle market.

In the dynamic world of bottle manufacturing, the choice between Polypropylene (PP) and Polyethylene Terephthalate (PET) is an intricate balancing act. Each material boasts its own set of strengths and limitations, serving as a testament to the diverse landscape of modern packaging.