Why does PVC resin and recycled materials cause the "dripping" phenomenon on the surface of products?

During the processing and production of PVC products, many customers often encounter a confusing and troublesome problem: when producing PVC products such as profiles, pipes, foam boards, or sheets, obvious "dripping" or "water seeping" phenomena occasionally appear on the surface of the finished products. Some customers even reflect that even if they have specially used dehumidifiers to dry the raw materials in advance, and strictly controlled the moisture content of PVC resin and auxiliary materials, the surface of the produced products still has a sticky, wet feeling, or even obvious small water droplets, which seriously affects the surface finish, appearance quality and product performance of the products. In severe cases, it may even lead to product scrapping, increasing production costs and affecting delivery progress.

In fact, this "dripping" phenomenon that occurs on the surface of PVC products during processing or use is not a real "water seepage" in the traditional sense, nor is it caused by incomplete drying of raw materials in most cases. It is a common surface problem in PVC processing, which is also known in the industry as precipitation phenomenon, condensation phenomenon or product surface wetting phenomenon. This phenomenon is not only related to the processing technology, but also closely related to the type of raw materials, the matching of auxiliary agents, and the environmental conditions during production. It often occurs in the processing process of PVC resin, especially when using recycled PVC materials (recycled materials), because the composition of recycled materials is more complex, which is more likely to trigger this problem.

Many customers have misunderstandings about this phenomenon, thinking that it is only caused by the high moisture content of raw materials, so they only focus on strengthening the drying of raw materials, but ignore other potential causes, resulting in the problem not being solved fundamentally. In order to help customers thoroughly understand and solve this troublesome problem, avoid unnecessary losses, and improve product quality and production efficiency, we will conduct a detailed analysis of the possible causes of the "dripping" phenomenon on the surface of PVC products below, and clarify the key factors affecting this phenomenon, so as to provide targeted solutions for customers.

·Problems with the PVC resin itself

①Residual low-molecular-weight substances

Unreacted monomers (VCM), oligomers or plasticizers remaining in the production process of PVC resin will migrate to the surface of the product during high-temperature processing or long-term use, forming oily precipitates (similar to water droplets).

How to solve it: Select high-purity PVC resin (such as suspension resin, with residual VCM < 1ppm)

②Insufficient thermal stability

PVC is prone to decomposition during processing, generating HCL gas. If insufficient heat stabilizers (such as common lead salt stabilizers or calcium-zinc stabilizers) are added, the decomposition products can easily combine with water to form acidic droplets.

How to solve it: Appropriately increase the dosage of the heat stabilizer or compound a more efficient stabilizer.

·The inherent defects of recycled materials

①Impurities and degradation products

The recycled materials contain waste additives (such as aged plasticizers), fillers (calcium carbonate), or contaminants (oil stains, dust, etc.), which are prone to leach out during secondary processing.

How to solve it: Strictly clean the recycled materials and add adsorbents (such as activated carbon) to remove impurities.

②Molecular chain breakage

Multiple processing leads to the degradation of PVC molecular chains, generating more low-molecular-weight substances and accelerating the migration rate.

How to solve it: Limit the proportion of recycled materials added and supplement toughening agents (such as CPE) at the same time.

·Formula and process factors

①Excessive plasticizer or migration

Phthalate plasticizers (such as DOP) are prone to exudation, especially in high-temperature and high-humidity environments, where the surface shows a "sweating" phenomenon.

How to solve it: Switch to high-molecular-weight plasticizers or polyester-based plasticizers.

②Lubricant imbalance

Excessive external lubricants (such as PE wax) will precipitate, forming white powder or oil droplets.

How to solve it: Optimize the lubrication system (the ratio of internal and external lubrication), such as compounding oxidized polyethylene wax (OPE wax).

③The processing temperature is too high

When the temperature exceeds 190℃, the decomposition of PVC intensifies, and the volatile substances condense and adhere to the surface.

How to solve it: Control the extrusion/injection molding temperature (160-180℃), and use low-temperature foaming agents (such as NC foaming agents).

·Environmental factors

①The influence of humidity

PVC products absorb moisture in a humid environment, and additives (such as CaCO3) may seep out water after absorbing moisture.

How to solve it: Add desiccant or surface coating treatment.

②Temperature difference condensation

When products are exposed to alternating cold and hot environments (such as refrigerated packaging), the condensation water on the surface is mistakenly regarded as a "dripping" phenomenon.

How to solve it: Improve the air permeability of the packaging or use an anti-condensation formula.

By systematically inspecting PVC resin, recycled materials, formulas and environmental factors, the "dripping" phenomenon of PVC products can be effectively reduced.