Isotactic polypropylene (iPP or usually just PP) is considered a crystalline commodity polymer of the polyolefin family. It has the following basic molecular structure: (CH2 – CHCH2).
Even though the molecule appears to have a bulky pendant group, there is substantial backbone mobility, allowing the molecule to twist in such a manner as to present a smooth and branchless albeit somewhat bulky molecule. Crystallinity is typically about 65% and crystallization rate is quite slow, probably due to the bulkiness of the molecule. PP crystallizes at about 0.4% the rate of HDPE, as measured at 30oC below their respective melt temperatures. Like polyethylene, PP is translucent in its unpigmented, unfilled state. The translucency is the result of crystallites or spherulites that are larger in dimension than the wavelength of light [0.4 to 0.7 micrometers], thereby interfering with light transmission. Since PP crystallization level is less than that of HDPE, light transmission is greater than that of HDPE. As a result, PP is said to have “contact transparency”. In addition to non-transparency, homo-PP has a relatively high glass transition temperature of about 5oC, and thus has marginal low-temperature [-40oC] impact strength.
PP is considered a competitor to impact polystyrene, PVC and APET in many thin-gage thermoforming applications. It is priced competitively in $/lb with the competition, but its 900 kg/m3 density is 83% of that of PS and two-thirds of that of PVC and APET. Its modulus, about 200,000 psi, is typically about half that of its competitors, at 400,000 psi. PP finds applications where the product must sustain high temperature [100oC] and aggressive environmental conditions.
Traditional PP, usually called homopolymer PP or just homo-PP, can be made quite viscous in the melt state, with fractional melt flow indices [MFIs]. But unlike HDPE, homo-PP has relatively little melt elasticity. It is well-known that thermoforming is basically a solid phase deformation process that depends on the elastic character of the polymer sheet [see J.L. Throne, Technology of Thermoforming, Hanser, 1996 for more information]. Typically, when homo-PP is heated to its melting temperature range of about 165oC, it moves from a stiff-rubbery solid to a floppy, syrupy liquid in only a few degrees, Figure 1.
It has been said that homo-PP thermoforming window ranges from a degree or two to nothing, depending on the former and the source of the polymer. “Solid phase pressure forming” or SPPF was introduced in the 1970s as a way of pressure-forming thin-gage homo-PP sheet into relatively simple shapes such as drink cups and unit dose cups. SPPF has been probably most successful in the production of hot-fill individual serving juice containers. Technically the process, described elsewhere [Throne, ibid., p. 710+], requires extremely careful monitoring and control of the sheet temperature during heating, and requires 50 psi or more air pressure in order to achieve adequate draw ratios. As a result, homo-PP has never been a widely accepted thermoformable material 1.
Modifications to Homo-PP
The earliest modifications to homo-PP to improve its melt strength or melt elasticity focused on copolymerization with ethylenic molecules to produce ethylene-propylene copolymers. Typically, EP copolymers have lower melting temperatures [about 158oC v. 165oC for pure PP], less ESCR, and greater cost, but superior sag resistance and good to excellent low-temperature brittleness.
Fillers were other early modifications to homo-PP to improve its hot strength. Talc, calcium carbonate and titanium dioxide [TiO2] at dosages of 10% (wt) to 20% (wt) yield improved stiffness at PP melt temperatures, as well as improved room temperature stiffness. It has been shown that, with appropriate coupling agents and compatibilizers, PP will accept filler loadings of 60% (wt) and in certain cases, even more. It must be remembered that fillers do not usually alter the morphological characteristics of the polymer, meaning that the melt temperature and glass transition temperature of homo-PP remain essentially unchanged by filling. If, for example, unfilled homo-PP low temperature impact strength is unacceptable for a given application, filled homo-PP will also be unacceptable. Typically, fillers do not lower the cost of the polymer. This is especially true with homo-PP, since there is a great disparity between the specific gravity of the homo-PP and the filler, and since homo-PP requires coupling agents in order to get the very smooth organic PP molecule to adhere to the inorganic filler.
Metallocene PPs seem to offer potential benefits, since their morphological structure apparently can be readily tailored to create higher melt strength, much like MDPEs. At this point, these polymers are relatively new and their costs are not yet in line with other modified PPs.
High-melt strength PPs have been under development for a few years now. Currently, it appears that Montell and Amoco have competitive resins available for thermoformers. The general characteristics of these PPs seem to be short-chain branching, much like HDPE. The short-chain branching yields much greater melt strength and therefore, less sag and much more controllable sag, than homo-PPs. There is an indication that the crystallites that are formed during cooling are smaller and more perfect than those obtained with homo-PPs. Smaller crystallites yield containers that are less opaque than homo-PP containers, and the containers are tougher.
As noted above, the crystallization rate of homo-PP is quite slow. Extensive work on nucleating PP to accelerate the crystallization rate shows that finer crystallites yield better stability of the PP after the sheet has been formed and is cooling on the mold surface. This has recently been verified by Millikan Chemicals Corporation’s development of Millad 3988, which yields a near-haze-free transparent container. The effect of sorbitol on crystallization rate of PP is usually shown as an increase in peak recrystallization temperature, as measured with Differential Scanning Calorimetry as the test specimen is cooling from the melt 2. This is seen in Table 1, taken from Millikan Chemicals datasheets. In addition to Millikan Chemicals, polymer suppliers such as Amoco, are working on or have developed similar nucleating adducts.
|Adduct||Xstal Temp||Loading Level|
|Dibenzylidene sorbitol||115oC||@ 1800 ppm|
|Methyl dibenzylidene sorbitol||120oC||@ 1800 ppm|
|Millad 3988||121oC||@ 1200 ppm|
|Dibenzylidene sorbitol||105oC||@ 1800 ppm|
|Methyl dibenzylidene sorbitol||107oC||@ 1200 ppm|
|Millad 3988||108oC||@ 600 ppm|
The Heating of PP
As already noted, PP requires careful heating, no matter whether the polymer is the basic homopolymer or a highly modified one. If we consider the nominal polymer forming temperature to be 165oC, the amount of energy required to raise PP from room temperature of 25oC to a nominal forming temperature of 165oC [140oC temperature increase] is about 100 cal/g or about 180 Btu/lb. However the energy uptake is definitely nonlinear. It requires only 50% of that energy [about 50 cal/g or about 90 Btu/lb] to heat to 130oC [a 105oC temperature increase, or about 75% of the total temperature increase], and the additional 50% to heat the remaining 25% of the way to the forming temperature. Furthermore, it only requires 65 cal/g or about 120 Btu/lb to heat PS or APET to the same forming temperature [65% as much energy] and only 55 cal/g or about 100 Btu/lb to heat PVC to the same forming temperature [55% as much energy]. This means that if the heating cycle for more traditional amorphous polymers [PS, APET, PVC] is optimized, the processing cycle for PP will need to be increased by as much as 50% 3. Of course, increased heating time can be accomplished by using an auxiliary preheater. This unit is placed between the take-off station and the initial engagement of the pin-chain rails.
Keep in mind that the energy added to the sheet during heating must be removed during cooling. Thus if the heating cycle is longer for PP than for amorphous polymers, the cooling cycle, too, must be longer 4. This point will be discussed shortly.
The final thermal conditioning of the sheet is extremely delicate for all but the most highly filled PPs. The primary concern is uncontrolled and uncontrollable sag. Sag bands are continuous polyfluorocarbon-coated wires that are slaved to the rail and index when the rails index. They are used where appropriate. PP is quite sticky when heated above its melt temperature and care must be taken to ensure that the sheet releases cleanly from the sag band as it is indexed into the mold region. Even momentary sticking can cause excessive webbing, non-uniform wall thickness distribution, and rejected parts. Typical forming cycle times are on the order of 3 seconds for 20 mil sheet to 7 seconds for 34 mil sheet.
Mold Design Considerations
As noted, PP is sticky and typically very weak when molten. The sheet can easily tear when contacted with a high-speed plug assist. Heated aluminum plugs seem to work best so long as the plug temperature is very carefully controlled. However, polyfluorocarbon-coated epoxy foam plugs, sometimes called syntactic plugs, are more popular, even though plug mark-off is noticeable.
Because solid PP is slippery-smooth, the mold surface should not be. Parts that are formed on molds with very smooth surfaces may exhibit a series of horizontal banded lines owing to the sheet alternately sticking, then slipping on the mold surface. Large, flat expanses on molds with very smooth surfaces will trap air between the PP sheet and the mold surface. Parts with air trap will have surfaces with characteristic dimples and highly shiny regions.
Grids or cavity isolators are requisite with PP to prevent excessive sheet drawing from one cavity to another. Although many small containers are made with flat lands or lip regions, peripheral cavity dams or moats and dams help minimize the slippery polymeric sheet from being drawn non-uniformly into the mold cavity. Coining or pressing the sheet between two metal surfaces is recommended if a perfectly flat, close-tolerance lip is needed, for heat-sealing for example.
Female molds are recommended with PP. PP shrinkage is about 3 to 8 times that of amorphous polymers such as PVC, PS and APET. Substantial draft [5o for smooth wall parts, more if the mold wall is textured] is required for PP forming on a male mold.
It is recommended that PP be formed against a heated mold, with the mold temperature being on the order of 100oC or so. As noted, the recrystallization temperature range is on the order of 110oC to 120oC. Cooling times become large at mold temperatures above this. Below this temperature, recrystallization may be so slow that the part is released from its fixture, the mold surface, before it has fully crystallized. The results of this early release are part distortion and warpage.
As noted above, cooling times for PP are about 50% longer than those for competitive amorphous polymers. Cooling efficiency therefore becomes paramount. No longer is it sufficient to cool the mold strictly through the mold bottom with a cooling plate. Cooling channels along the sides and into the rim area of deep parts are recommended. The placement of additional cooling channels implies additional tooling cost. Owing to the extreme temperature sensitivity of the recrystallization of PP, care must be taken to provide sufficient cooling channels. It is recommended that the temperature differential from inlet to outlet of a given cooling channel not exceed 3oC or 5oF. If the mold is substantially hotter in one region than elsewhere, differential recrystallization will take place, resulting in warpage and distortion not only of the part in that region, but in the web surrounding it, and by implication, the other parts attached to that web.
PP is a fiber former. That is, when PP is elongated locally, it draws rather than fracturing. As a result, every effort must be made to minimize pinching PP between shearing surfaces. For punch-and-die trimming, gaps between the mating tools need to be as close to zero as possible. For compression cutting, such as steel rule die cutting, the die must be kept as sharp as possible. Some molders recommend heated dies for sheet thicknesses of 25 mil or more. Although double hollow-ground beveled dies produce the cleanest trim, single bevel dies are used because they are less expensive and can be leather-stropped to work out feathering that can cause “angel hair” and fuzz. Trimming parts on an in-line canopy trimming press can be problematic, since the web is usually thicker than the molded part and therefore cools more slowly and without support. This means that although the parts are relatively warpage- and distortion-free, the cooling web may have distorted the entire sheet before it is fed into the registering lugs. This distortion will result in out-of-register parts and a substantial generation of unusable parts before appropriate adjustments are made.
In-situ trimming or trimming on the mold becomes difficult with PP, primarily because the trimming area is usually thicker than the rest of the part and therefore, at the time of trimming, it may not have achieved the same degree of crystallization as the rest of the part. Once the part is cut away from the web, support is lost and the trim area is free to distort. And it usually does.
PP thermoformers have learned that PP requires better process controls and more attention to mold design and trimming. And this points to higher quality operators and maintenance people. According to Biller [see footnote 3], Those thermoforming companies operating their [PP] equipment efficiently and progressively are usually the market leaders in quality and performance. The higher the recrystallization temperature, the more rapidly crystallization is taking place and typically, the smaller the final crystallites will be.
1. * This doesn’t mean that homo-PP cannot be thermoformed successfully. Amoco and Brown Thermoforming Machinery demonstrated thermoforming of thin-gage unit dose cups from 48-inch wide sheet at the 1976 NPE show in Chicago.