Polypropylene (PP) has excellent mechanical properties and is widely used in many fields. However, due to the polymerisation process (e.g. catalyst, type of copolymerisation monomer), additive components (e.g. antioxidants, etc.), and the processing process (e.g. the degree of screw shear, processing temperature, etc.), modified PP materials often have high VOC and severe odour, making it difficult to satisfy the demands of automotive interior use.
Common plastics modification enterprises to control the odour and VOC content of PP materials to preferred low-odour PP raw materials, plus additives (such as complex antioxidants, physical and chemical adsorbents, odour masking agent, etc.) method is the main, and with the elimination of the process (such as the extrusion process of negative pressure operation, material drying, etc.) to improve the odour problem.
Commonly used adsorbents are divided into two categories of chemical and physical adsorption, which is mainly through the specific or non-specific adsorption process of small molecules of the odour, to achieve a chemical reaction with the small molecules and produce a larger molecular weight and difficult to volatilise another compound, or physically bound to achieve the effect of elimination of the odour. However, these two methods also exist a single type of chemical reaction, high cost and adsorption capacity is limited, the addition of a large amount of problems, often limited deodorant effect. In addition, there are also by adding a small amount of masterbatch enriched with fragrance, which is used to cover the resulting unpleasant odour, but by itself it only covers the unpleasant odour and does not effectively improve the concentration of the gas, and there is also the problem of incomplete covering.
Therefore, for the odour problem in the process of modified PP, this paper proposes the methods of step-by-step mixing of raw materials and post-processing of modified materials, respectively, by adjusting the mixing order of the original materials, using extraction solvents and cooperating with high-temperature de-volatilisation process, to remove low molecular volatiles on the surface of modified PP and its inner part after granulation processing, so as to achieve the purpose of low odour and low VOC.
Experimental part
1.1 Raw materials
Polypropylene A: ethylene – propylene copolymer, 230 ℃, 2.16kg under the conditions of the melt mass flow rate (MFR) of 20 ~ 50g/10min,
Polypropylene B: propylene homopolymer, 230 ℃,2.16kg under the condition of MFR is 10 ~ 30g/10min,
Antioxidant 3114, antioxidant 168, antioxidant 1024: industrial grade,
Talcum powder: KCM-6300, 2000~3000 mesh,
Odour adsorbent: QL-A, porous silica-aluminium inorganic and organic mixture,
Ethanol, acetone, ether, calcium stearate: industrial grade,
1.2 Equipment and apparatus
1.3 Preparation of specimens
The effects of different copolymer polypropylene and homopolymer polypropylene raw material ratios, different mixing methods and material post-treatment methods on the mechanical properties, odour grade and VOC content of the modified polypropylene were investigated respectively. Among them, the step-by-step mixing method, i.e., polypropylene and antioxidant were mixed to obtain mixture S1, respectively; black masterbatch, talc, deodorant, and calcium stearate were mixed to obtain mixture S2, and finally S1 and S2 were mixed and extruded for pelleting.
Solvent post-treatment mode, that is, after the completion of the material granulation surface spraying mass percentage concentration of 50% post-treatment solvent (taking into account the actual production safety requirements, the configuration of the components in the solvent, selected components volume ratio of ethanol: ethyl ether: acetone: water = 3: 1: 1: 5), according to the proportion of 10mL of each kg of granular material spray, and then mixed and stirred well at room temperature and static for 0.5 ~ 1h.
1.3.1 Modified PP formulation design under different copolymer polypropylene and homopolymer polypropylene mass ratio
Polypropylene A, polypropylene B, antioxidant 3114, antioxidant DSTP, antioxidant 1024 in accordance with the proportion of the formula in a high-speed mixer dry mixing for 3 ~ 5min, and then removed and set aside to obtain the first mixture S1. At the same time, the ferrous masterbatch, talc, odour adsorbent, calcium stearate in accordance with their respective proportions in a high-speed mixer dry mixing for 3 ~ 5min, and then added to the step before the obtained first mixture S1, continue mixing for 3~5min, mixing temperature 30~40°C, get the second mixture S2, the second mixture S2 in the twin-screw extruder by melting, mixing, extrusion and granulation, get granular material S3.
The specific processing process is as follows: 180~190°C in the first zone, 200~210°C in the second zone, 200~210°C in the third zone, 200~210°C in the fourth zone, 210~215°C in the fifth zone, 210~215°C in the sixth zone, 215~215°C in the seventh zone, 215~225°C in the eighth zone, 215~225°C, with a residence time of 1~2min, a pressure of 15~18MPa, and a vacuum degree of -0.1~-0.2MPa.
The granular material S3 obtained by spraying the mass percentage concentration of 50% of the post-treatment solvent (ethanol: ether: acetone: water volume ratio = 3:1:1:1:5), according to the proportion of 10mL per kilogram of granular material spraying, mixing and stirring uniformly at room temperature and static 0.5 ~ 1h, then placed in a 100 ℃ oven, the speed of the fan is 2500r/min, the atmosphere of nitrogen, baked after 12h. That is to obtain low odour, low VOC polypropylene composites. The specific formula design is shown in Table 1.
1.3.2 Formulation design of modified PP under different mixing and post-treatment methods
In order to explore the odour effect of modified polypropylene under different treatment methods, different mixing methods and post-treatment methods were designed with reference to the ratios of the raw materials in formula 1# and compared. 6#-8# of the specific formula design is shown in Table 2.
1.4 Testing and characterisation
Results and Discussion
2.1 Influence of the composition of polypropylene raw materials in the formulation on the mechanical properties and odour of modified PP
Due to the needs of the actual processing and use of automotive interior products, a small amount of inorganic substances (such as colour powder, filler, glass fibre, etc.) are often added to the product to carry out physical mixing to improve its colour, heat resistance, hardness, rigidity, shrinkage, etc. Due to the poor direct interaction between inorganic fillers and resins, the toughness of the product tends to decrease more significantly after adding, and it can’t meet the use of the demand. Therefore, according to the actual use of demand, in the formulation design of impact copolymer polypropylene A and homopolymer polypropylene B for compounding, in order to meet the excellent processing fluidity and rigidity of the material at the same time, give the material a certain degree of impact toughness to meet the use of most of the automotive interior parts product demand. According to the experimental needs we were adjusted copolymer polypropylene and homopolymer polypropylene mass ratio (the total amount of 100 parts) for 1:1, 1.3:1, 1.5:1, 2:1, to explore its effect on the mechanical properties and odour of modified PP. The specific formulation design is shown in Table 1.
In terms of mechanical properties, comparing the results of 1#, 3#, 4# and 5#, it can be seen that the toughness of modified PP increased with the increase of copolymerised polypropylene content, and the cantilever beam unnotched impact strength increased from 52.3kJ/m2 to 78.1kJ/m2 respectively (as shown in Fig. 1a), however, there was a significant decrease in the rigidity and strength of the material, such as flexural modulus, tensile strength, and so on. The bending modulus decreased from 2645 MPa to 1924 MPa (as shown in Fig. 1b), respectively. The processing performance of the material also changed slightly, but the MFR was still basically maintained at about 10-14.5 g/10min (as shown in Fig. 1c). This also indicates that the effective adjustment of the stiffness and toughness properties of the modified PP complex system can be achieved by adjusting the ratios of copolymerised polypropylene and homopolymerised polypropylene. In addition, comparing the experimental results of 1# and 2#, it can also be seen that the overall stiffness of the material increases significantly and the toughness decreases more obviously when the filler is added in larger amounts. This is due to the fact that when a small amount of talc is added, it has a heterogeneous nucleation effect, which can promote the formation of polypropylene α crystal type and improve the rigidity of PP. However, when a large amount is added, it is mainly physical filling, while its distribution uniformity in polypropylene is limited, resulting in a significant decrease in impact properties. In addition, the addition of a large amount of talc also leads to an increase in the density of the product and a decrease in the processing performance (MFR is only 8.9g/10min, as shown in Fig. 1c), which is also not in line with the future development trend of automotive lightweighting.
Due to the strong thermal shear effect during polypropylene modification, the material is prone to degradation when melting and extruding, and produces more low molecular organic compounds (such as aldehydes and ketones), which have a greater impact on the final odour level and the safety of air quality inside the car. In addition, in October 2011, the GB/T27630-2011 “Guidelines for the Evaluation of Air Quality in Passenger Vehicles” clearly listed the list of carcinogenic substances (including benzene, toluene, formaldehyde, xylene, ethylbenzene, acetaldehyde, acrolein) for control in automobiles.
Therefore, we subsequently analysed the VOC content and odour levels of each experimental group. The experimental results in Table 3 show that adjusting the ratios of copolypropylene and homopolymer polypropylene had an effect on improving the overall VOC and controlling the odour class, with an increase in copolypropylene content slightly increasing the overall VOC content, the odour class from 3 to 3.2, and the VOC content from 29.55 to 32.44 μg/g. This is due to the fact that copolymer polypropylene polymerisation in the production process, the introduction of the second or third component (e.g., C4 component such as butene) often leads to an increase in the odour small molecules in the product, while different purity raw materials make the impurity gases in the total system also increase, which also affects the odour grade of the final material. However, taken together, the difference in odour between the parallel groups is not that significant. In addition, the overall odour component is higher in aldehydes and ketones compared to non-polar aromatic hydrocarbons, which is due to the fact that aldehydes and ketones are mainly produced during the processing of modified polypropylene. Therefore, reasonable adjustment of processing parameters (e.g., temperature, material residence time) together with appropriate antioxidant components is beneficial to control the overall odour level of the system. Meanwhile, comparing 1# and 2#, it can be seen that the odour level of the material is also reduced when a large amount of talc is filled, which is due to the lamellar structure of talc, which has a certain non-specific adsorption and physical barrier effect, and it can prevent the overflow of odorous small molecules to a certain extent, thus improving the odour level of the modified PP, but the improvement ability is limited, and at the same time, there is a large loss of some of the mechanical properties. Therefore, the content of homopolymer and copolymer polypropylene can be adjusted in the modification process to meet the actual performance of the products, while it will not have too much effect on the odour of the final modified PP. So for the later experiments, we choose the 1# formulation with stiffness and toughness balance as the basis to continue the experiments.
2.2 Analysis of the source of odour in the formulation
Taking 1# formula as the basis, under the condition that other components remain unchanged, by removing the ethylene-propylene copolymer A, propylene homopolymer B, black mother and talc group in the formula in turn, we conducted the temperature level measurement and VOC test experiments, exploring the influence of each component on the source of odour in the formula, and the specific results are as follows.
Comparison with the results of the 1# experiment shows (e.g. Table 4) that the presence of different components in the formulation has a greater effect on the odour grade and VOC content of the modified polypropylene, compared to the homopolymerised polypropylene, when the amount of copolymerised polypropylene in the masterbatch is reduced, the overall odour improves and the VOC level slightly decreases (from 29.55 μg/g to 28.03 μg/g), which is due to the fact that the copolymerised Polypropylene A was prepared by gas-phase polymerisation process, the viscosity of the system increased during the copolymerisation stage, and the diffusion resistance of odorous low molecules into the rubber phase increased, resulting in an increase in odour. However, the overall effect of copolymerisation and homopolymerisation on the odour of the system is not significant, as the de-volatilisation process at the later stages of the production process removes most of the odorous small molecules. Meanwhile, in comparison, the addition of black masterbatch had a greater impact on the odour of the modified polypropylene, and the removal of black masterbatch resulted in a significant improvement in odour, with the VOC content decreasing from the previous 29.55 μg/g to 21.66 μg/g, and a more pronounced decrease in the volatile components. This is due to the black masterbatch in the preparation process due to the source of carbon black components, carrier resin source, antioxidant addition, processing temperature, lubrication and dispersant type can lead to a great difference in the odour, coupled with carbon black masterbatch for the antioxidant component of the system adsorption, will also lead to modified polypropylene’s heat and oxidation resistance to decline, so a reasonable choice of black masterbatch type for the improvement of the overall odour level has a greater Therefore, a reasonable selection of the type of black masterbatch is more helpful to improve the overall odour level. In addition, the presence of talc is helpful in improving the odour of the modified polypropylene, similar to the principle of odour improvement in the previous 2#.
2.3 Effects of different treatments on the odour and mechanical properties of modified PP products
Subsequently, we further explored the mechanical and odour effects of modified PP with the same formulation composition and different mixing and post-treatment methods. From the experimental results in Fig. 2, it can be seen that the tensile strength, flexural modulus, and impact strength of each group of modified PP fluctuated, but the overall mechanical properties did not differ much, and all of them had a better stiffness-toughness balance characteristics. At the same time, the processing properties between the groups of modified PP are also basically similar, and the MFR is basically around 12-13g/10min. It is implied that even with the addition of one step mixing process or post-treatment process, the related additives (e.g., antioxidants) in the modified materials did not undergo a more significant loss. Therefore, the experimental results also indicate that the simple mixing method of the original material and the post-treatment method of simple solvent after granulation do not have a large impact on the mechanical properties of the final modified PP, which is also beneficial to the practical operation in the production process.
The differences in odour and VOC of modified PP in each group using different mixing methods and post-treatment were further compared. As can be seen from the experimental results in Table 5, comparing the results of 1# and 8#, 6# and 7#, the overall VOC and odour grades of modified PP were lower after the stepwise mixing operation, which indicates that the stepwise mixing step is also useful for controlling the VOC concentration and improving the odour. Among them, the content of non-polar volatiles (e.g., benzene, toluene, ethylbenzene, xylene) in the modified materials did not change much, and the content of aldehydes and ketones changed more significantly, with the acetone content decreasing from 12 μg/g to 10 μg/g, and 18 μg/g to 16.5 μg/g; and the acetaldehyde content decreasing from 5.7 μg/g to 3.1 μg/g, and 5.5 μg/g to 5.1 μg/g. This is attributed to the fact that, through the process of first PP and the antioxidant is fully mixed to increase the content of antioxidant in polypropylene, to avoid the problem of the decrease in the anti-thermal oxidation effect of PP due to the adsorption of antioxidant after the addition of the filler talc and masterbatch, and to ensure that the prepared polypropylene resin maintains a better thermal stability in the processing and use process, which effectively reduces the source of odour in the processing of polypropylene resin (e.g. small molecule ketones, acids, and alkanes generated by the degradation). etc.). At the same time, taking into account the common colour masterbatch use process there is a big smell, volatiles, the first fully mixed with the adsorbent and talc, through the two adsorption and barrier effect, minimize the volatile organic volatiles emitted, and appropriate addition of calcium stearate as a lubricant and acid binding agent, improve the dispersion of the various inorganic components in the main polypropylene, absorbing thermal shear generated by acidic small molecules The effect of the polypropylene is to effectively improve the processing stability of polypropylene and ultimately improve the odour effect of the material. Therefore, it effectively reduces the aldehydes and ketones produced by thermal degradation during processing, and has a better effect on the improvement of final odour.
Meanwhile, comparing the results of the odour experiments of 1# and 6#, 7# and 8#, it can be seen (as shown in Table 5) that the VOC content was reduced from 35.23 μg/g to 29.55 μg/g, and 41.34 μg/g to 34.57 μg/g, and the odour grade was also reduced from 3.5 to 3, and 4 to 3.3, respectively, and the acetone content was reduced from 16.5 μg/g to 10 μg/g and The acetone content was reduced from 16.5μg/g to 10μg/g and 18μg/g to 12μg/g, which also shows that the use of post-treatment agent can further reduce the concentration of small molecule volatiles, VOC content and improve the odour level, no matter it is a step-by-step mixing or mixing operation of all materials together. A side-by-side comparison shows that the improvement ability of the post-treatment agent is more obvious and superior to the stepwise mixing process. This is due to the fact that the method is similar in principle to the common vapour extraction jellyfish pellets, which use small molecule extractants or low-boiling point solvents to achieve odour molecule removal. The specific principle is to add the appropriate concentration of low-boiling point organic volatile solvents for the extraction process, the process can be efficient and rapid dissolution and extraction of the modified material surface, shallow and pore residual small molecules of odour, to accelerate the small molecules of volatiles in the interior of the material to migrate to the surface of the modified material to reduce the internal and surface residues. Finally, the odour small molecules are extracted and taken away by high temperature baking and N2 blowing process. However, the difference is that the concentration of solvent and the extraction time (0.5-1h of standing) are more controllable in this process. Compared with the common vapour-extraction type jellyfish pellets which have less organic components (higher water content), shorter residence time in the screw and larger additive amount, resulting in limited extraction capacity of organic volatile components in the melt, the solvent post-processing method is more efficient and simple, and therefore can better improve the odour effect of modified PP, and ultimately achieve the purpose of low odour and low VOC.
Conclusion
1) By adjusting the content of homopolymer polypropylene and copolymer polypropylene, the stiffness and toughness balance characteristics of modified polypropylene can be adjusted to a certain extent to meet the needs of different automotive interior parts.
2) By adjusting the mixing method of polypropylene with various components, such as additives, fillers, masterbatch, etc., the generation of small molecules during polypropylene processing and the influence of the final odour grade can be effectively controlled.
3)By further using a certain concentration of low boiling point volatile solvent after modifying the material, it can efficiently and rapidly dissolve and extract the residual small molecules of odour on the surface of the modified material, in the shallow layer and in the pores, reduce the internal and surface residues, and effectively improve the odour effect of the modified PP.