Recycled PET sheet extrusion often faces three major quality problems: yellowing, poor toughness, and insufficient gloss. These problems become more noticeable when using a high percentage of recycled PET flakes or regrind. For manufacturers producing PET thermoforming sheets, these defects can affect product appearance, forming performance, and final product quality.
1. Why Does Recycled PET Sheet Turn Yellow?
PET sheet may turn yellow due to thermal degradation, moisture-related hydrolysis, contaminated recycled materials, or excessive residence time at high temperatures. Proper raw material management and optimized extrusion conditions are essential.
For parallel twin screw PET sheet extrusion lines, appropriate screw configuration, temperature control, vacuum degassing, and effective melt filtration can help improve melt quality and reduce processing defects.
Recycled PET sheet color and toughness before and after twin screw extrusion optimization
2. How to Improve PET Sheet Toughness and Gloss
Poor toughness may result from PET molecular weight degradation, reduced intrinsic viscosity (IV), or inconsistent recycled material quality. Insufficient gloss may be related to contamination, melt uniformity, die conditions, or cooling roller settings.
In addition to optimizing the extrusion process, manufacturers can consider the following masterbatch formulations as starting points for production trials.
| Masterbatch Type | Recommended Addition per 1,000 kg of PET |
|---|---|
| Anti-yellowing masterbatch | 300-600 g |
| Toughening / anti-cracking masterbatch | 6-8 kg |
| Whitening masterbatch | 1.5 kg |
| Gloss-enhancing masterbatch | 1.5 kg |
These dosages are reference formulations and should be validated according to the masterbatch supplier’s specifications, recycled PET quality, and required sheet properties. Whitening and gloss-enhancing masterbatches can improve appearance, while toughening additives may help improve impact performance. Actual results depend on additive compatibility and processing conditions.
3. How Parallel Twin Screw Extrusion Improves Recycled PET Sheet Quality
A properly designed parallel twin screw PET sheet extrusion line can provide effective mixing, degassing, and processing of suitable recycled PET materials. Combined with appropriate filtration, temperature control, and accurate additive dosing, it can help manufacturers improve sheet color consistency, toughness, and surface appearance.
The optimal equipment configuration and processing parameters depend on recycled material composition, target output, sheet thickness, and final application. Maintaining stable raw material quality and controlling PET degradation are also essential for consistent production results.
FAQ
Por que é que as chapas de PET reciclado ficam amareladas?
Destacam-se quatro causas: degradação térmica, hidrólise relacionada com a humidade, material reciclado contaminado e tempo de permanência excessivo a altas temperaturas de fusão. O amarelecimento é uma alteração química na cadeia polimérica, pelo que é melhor controlá-lo a montante, através da gestão da matéria-prima, do controlo da temperatura, da desgaseificação a vácuo e da filtração da massa fundida.
É possível reverter o amarelecimento depois de a folha ter sido produzida?
Não. O amarelecimento reflete uma alteração molecular no próprio PET, pelo que não pode ser removido através de polimento ou lavagem posteriormente. A solução prática é a prevenção: manter os flocos secos e limpos, reduzir o tempo de permanência e adicionar um masterbatch anti-amarelecimento numa dosagem validada.
Por que razão as chapas de PET reciclado perdem tenacidade e resistência ao impacto?
A perda de resistência surge normalmente na sequência de uma diminuição do peso molecular e da viscosidade intrínseca (VI). As escamas recicladas chegam com valores variáveis de VI e diferentes históricos de exposição térmica, pelo que a massa fundida pode conter frações degradadas que se partem sob impacto. Uma qualidade consistente das escamas e um processamento térmico suave protegem a VI.
Qual é a quantidade de masterbatch anti-amarelecimento utilizada por tonelada de PET?
A common starting point is 300-600 g per 1,000 kg of PET. The exact level depends on the masterbatch supplier’s specification, the color of the incoming recycled flakes, and the target sheet appearance, so it should be confirmed in production trials.
Qual é a quantidade de masterbatch de reforço adicionada à chapa de PET reciclado?
A dosagem de referência é de 6-8 kg por 1 000 kg de PET, o que corresponde aproximadamente a dez vezes a dosagem do aditivo anti-amarelecimento. O desempenho em termos de impacto depende da compatibilidade do aditivo com a resina de base e das condições de processamento; por isso, valide a dosagem na sua própria linha de produção antes de aumentar a escala.
Uma linha de dupla rosca necessita de pré-secagem para lascas de PET reciclado?
Vacuum degassing and multi-vent design remove moisture and volatiles during extrusion, which reduces the crystallization and pre-drying burden that a linha de extrusão de folha PET de parafuso único normally requires. Extremely wet or heavily contaminated flakes may still need upstream conditioning.
What causes poor gloss on recycled PET sheet?
Insufficient gloss usually traces to contamination, poor melt uniformity, die conditions, or cooling roller settings. Stable melt pressure, clean filtration, and controlled cooling all have a direct effect on surface finish.
Can an ABA co-extrusion structure hide recycled PET discoloration?
Yes. An ABA PET three-layer co-extrusion line places virgin outer skins over a recycled core, so slight core discoloration is much less visible on the finished product. It is a practical option when recycled content is high but surface appearance must stay premium.
How can sheet producers keep recycled material quality consistent?
Close the loop in-house. A triturador online de sucata de chapa returns skeletal waste directly to the extruder, so the regrind stream stays clean, dry, and traceable instead of accumulating unknown contamination.