Application and Development of Polypropylene for Thermoforming in the Packaging Field

1. Basic principles and characteristics of thermoforming

Thermoforming is a special plastic processing method to process thermoplastic sheets into various products. The thermoplastic sheet is heated to soften and then pressurized (vacuum or positive pressure) to make the thermoplastic sheet act on the surface of the mold to obtain a shape similar to that of the mold surface, and after cooling and shaping, the product is trimmed to obtain the desired shape. Thermoforming is fast and uniform, with short molding cycle time and low mold cost, suitable for automation and long production time, and is considered to be the most efficient processing method with the lowest unit cost among plastic molding methods. Especially with the continuous improvement of thermoforming process and equipment and the intervention of computer simulation, thermoforming has become the main processing method competing with injection and blow molding. In recent years, new advances have been made in thermoforming processing equipment. Jimin has introduced the thermoforming equipment and processes exhibited at K2013 in Germany, where speed, versatility, control technology and cost reduction have become key themes for new equipment for thermoforming in packaging and industry.

The main types of thermoforming are: vacuum forming, pressure forming, plunger-assisted forming and combinations of them. There are more and more thermoformed products in the market, such as cups, plates, food trays, toys, caps and helmets, home appliance parts as well as automotive parts, architectural decorative parts, chemical equipment, etc. Compared with injection molding, thermoforming has the advantages of high production efficiency, low investment in equipment and the ability to manufacture products with large surface areas. The main plastics used in thermoforming are polystyrene (PS), polyvinyl chloride (PVC), polyolefins such as PE, PP, polyacrylates (PMMA) and cellulose such as nitrocellulose (NC), cellulose acetate (CA), etc. Engineering plastics such asacrylonitrile-butadiene-styrene (ABS)are also used. Styrene plastic (ABS), polycarbonate (PC), etc. The development of high performance PP such as high transparency, high melt strength and low warpage has promoted the application and development of PP in the field of thermoforming.

2.　Domestic Thermoforming Packaging Market Status

There are more than 3,000 domestic thermoforming packaging companies, of which more than 300 have an annual output of over 1,000 tons. About 70% of the factories with annual production capacity of thousands of tons are self-produced sheets and integrators of thermoformed products. PS (OPS) and polyethylene terephthalate (PET) are the main outsourced sheets, while some capable factories produce their own PP, PS and PVC sheets and small factories outsource their sheets. Thermoformed products are mainly used in food packaging, non-food packaging and cut sheets, and the requirements for raw materials vary from application to application.

3. Development of PP for thermoforming

Domestic PP thermoforming applications started in the 1990s and have been developing rapidly in recent years, with an annual growth rate of about 10%. Now there are many large and famous thermoforming plastic packaging manufacturers in China, such as Japan Central Chemical, who have built plants in China and put them into production, and their products are mainly focused on medium and high-end plastic thermoforming packaging products. At present, there is a lack of special PP materials specifically for thermoforming process in China. Domestic thermoforming products manufacturers generally purchase T30S and other general-purpose resin modified for use, but the products obtained through post-modification and industrial scale production of products, there are uneven additive control, tensile properties do not meet the requirements, uneven container wall thickness in thermoforming, extrusion coating calendering edge curling and other problems, it is difficult to meet the high quality and high requirements of the application requirements, but also It is difficult to meet the demand of high quality and high requirements, and also restricts the production efficiency of downstream processing enterprises.

Overseas thermoforming special materials are mostly homopolymers, transparent random copolymers are less, and some grades have long import cycles, so domestic downstream thermoforming manufacturers rarely use imported materials. The existing sheet production of thermoforming product manufacturers is mainly 600 mm wide sheet, with the progress of the molding machine technology, the width of the sheet is increasing, the widest can be more than 1000 mm, so the sheet’s anti-melting ability to put forward higher requirements. The solution in the downstream products industry is to add a certain percentage of high-density polyethylene (PE-HD) to ordinary PP to improve the sheet’s melt sag resistance, but this method will bring uneven mixing and limited transparency of products. The domestic thermoforming industry has an increasingly urgent demand for PP raw materials with high performance and ultra-high transparency.
Before 2012, the research and development of high transparent thermoforming materials in China was relatively lagging behind. At that time, the R&D situation of domestic thermoforming special materials is listed in Table 2.

None of these specialty materials have been further worked on for the higher melt strength and wider molecular weight distribution required for thermoforming processing, and some have not been produced on a large scale.

In recent years, Tianjin Petrochemical has continued to develop special materials in the field of PP thermoforming, and achieved significant economic and social benefits, adding color to this niche area of thermoforming.

Table 4 shows the special PP materials for thermoforming developed by several petrochemical plants in recent years, which are developed by Shanghai Ru Xia New Material and each petrochemical plant for different demands in different thermoforming segments. The downstream thermoforming product manufacturers are widely distributed, the industry concentration is low, there are few enterprises above the scale, and the demand for downstream products is diversified, and the petrochemical plants have their own plans for scheduling production, so there are still some difficulties to coordinate well with the upstream and downstream, but the development direction of the thermoforming industry is to make differentiated downstream products on the one hand, and to use special materials for production on the other hand, in order to improve product quality and improve the stability of product quality. The development and application of special materials for thermoforming is still promising.